Amplifier



June 5, 1928.

- 1,672,840 D. M. TERRY AMPLIFIER n Filed y 1924 2 Sheets-Sheet 1 @40 5 FEB as i fi -flw A? mall/0r ms /'7 .6 5 y flomld/Uerry June 5, 1928."

D. M. T ERRY AMPLIFIER m Filed May 8, 1924 l 2 spans-Sheet 2 5 a p 5 Z 4 hymn/:- v papa/4M 7hr Patented June 5, 1928-.

UNITED STATES 1,672,840 PATENT OFFICE.

DONALD M. TERRY, OF NEW YORK,N. Y., ASSIGNOR TO WESTERN ELECTRIC CO]!!- PANY, INCORPORATED, OF NEW YORK, N. Y., AQOIRPORATION OF NEW YORK.

AMPLIFIER.

This invention relates to amplifiers and more particularly to an electron discharge amplifier.

An object of the present invention is to amplify without distortion very small currents having component frequencies extending over a range from 'zero to several hundred cycles. 7

Another object is to amplify current of 10 all the essential frequencies produced by a photoelectric cell during the process of transmitting pictures by electricity at commercial speeds. 1 g

In one embodiment of the invention, an

electron discharge amplifier for amplifying current from a photoelectric cell is mounted inside of a metallic shield; For best results, the cathodes of both the amplifier and the photoelectric cell are operated at ground potential. This necessitates connecting the photoelectric cell battery directly, to the grid of the amplifier. A large resistance is required to couple the photoelectric cell and its battery to the input circuit of the amplifier because of the high internal impedance of the photoelectric cell and the very small amplitude of the current flowing therein. Special steps are taken to reduce the inherent capacity shunted across this resistance 80 in order to effect the amplification of all of the essenial component frequencies of' the photoelectric cell current.

The novel features which are considered characteristic of this invention are set forth with particularity in the appended claims.

The invention, both as to its organizatio and method of operation together wit other objects and advantages thereof, will be explained in the following description 40 having reference to the accompanying drawing consisting of the following figures: Fig.1 is a schematic circuit diagram of a photoelectric cell and amplifier according to this invention.

Fig. 2 is a front view of the assembled amplifier and photoelectric cell.

Fig. 3 is a top view of Fig. 2 with the cover removed showing the location of the batteries and other apparatus.

Fig. 4 is a vertical cross section of Fig. 2 looking toward the right from line 4-4 showing the location of the photoelectri cell battery.

Fig. 5 is a vertical cross section of Fig. 2

1924.' Serial No. 711,749.

waves 0 are connected to the input circuit of an electron discharge modulator M, the output circuit of which is connected to a transmission circuit L.

For further details of a system for transmitting pictures by electricity, to which the arrangement of Fig. 1 is applicable, reference may be had to a co'pending application of M. B. Long, Serial No. 681,347, filed December 18, 1923.

The photoelectric cell 5 is controlled by light from a source 8 passing through an elemental area of film 9 upon which it is focused by means of lens 10. The inside of the cell is coated with a light sensitive substance such as one of the alkali metals. Potassium or rubidium in rarefied neon gas forms a particularly sensitive cathode.

The photoelectric cell battery consists of three sections, so divided as to permit of coarse, medium and fine adjustment. The section 11 consists of five No. 764 Eveready batteries, each of 22.5 volts. are connected together in series aiding relationship and the terminals are connected' to the switch-points of switch 21. The positive terminal of the series arrangement is connected to the anode of photoelectric cell 5. The negative terminal of the last battery unit in use is connected through the switch arm of switch 21 to the positive terminal of battery section 12. Battery section 12 consists of six No. 734 Eveready batteries, each of 4.5 volts. The negative terminal of the last battery unit in use of battery section 12 is connected through switch arm of switch 22'to one terminal of potentiometer 15 which is connected by means of switch 26 across the terminals of battery 17 in series with resistance potential The individual batteries or units i 16. Battery 17 is a nine volt battery consisting of two No. 734 Eveready batteries.

By means of potentiometer 15, very fine gradations of potential may be obtained.

Switches 21 and 22 are so arranged that the battery circuit is never opened while cuttingv in or out battery units. Two switch arms are employed between which resistance 13 of 2,000 ohms is connected. When the switch arms are on adjacent switchpoints,

the "battery unit, the terminals of which are connected to these switchpoints, is not short circuited because of resistance 13. The batter 19 of 1.5 volts consisting of one cell of a 0. 734 Eveready battery determines the potential of the grid of amplifier 6. Potentiometer 18 connected across the input circuit of amplifier 6 is variable from 100,000 to 500,000 ohms in five steps. It. is preferably set at 200,000 ohms. The normal grid potential of amplifier 7 may be ad- 'usted by means of potentiometer 27. The

eating currents for electron discharge devices 6, 7 and 20 are controlled b rheostats 23, 24 and 25 and indicated y ammeters A A, and A Switch 29 controls the filament heating circuit of all the electron discharge devices. The output currents of devices 6, 7 and 20- are indicated by ammeters A,, A and A v The capacity between battery section 11 and the shield S is indicated by the dotted condenser 0.; that between battery section -12 and the shield by C that between the battery 19 and the shield by 0,. Capacity between the cathode and anode of the photoelectric cell is indicated by the dotted condenser G The effective capacity between the grid and cathode of device 6 is indicated by the dotted condenser C Referring now to Fig. 6, the effective in herent capacity between all of the equipment connected to the grid of the electron discharge device 6 and anode of photoelectric cell 5 and the shield S is represented b dotted condenser C. The value of this e fective inherent capacity is determined primarily by the capacities of condensers (1.,

C C and 0 all connected in parallel.

The condenser O is represented as being c'on-' nected between the upper end of potentiomb eter 18 and the shield S. The lower end of potentiometer 18 is connected to the oathode of electron discharge device 6 and the shield S. Consequentl the condenser C is connected in shunt o resistance 18.

In order to transmit the tone values of a picture faithfully, it is necessary that the current flowing through the photoelectric cell 5 have, at each instant, an amplitude corresponding to the given tone value involve This requires the changing of the current value proportionately and simultaneously with a change in tone value; When changing rapidly from a dark to a arrangement hereinafter to be describe can be transmitted in about four minutes.

If the speed is doubled, the numb'er of changes per second is increased to 1,000.

A condenser connected in shunt to a resistance increases the time required for the current to reach a steady state and therefore its maximum value in when a potential is suddenly impressed thereon. The greater the capacity of such condenser, the 'reater is the time required. The rate at w ich current can change in the resistance 18 of Fig. 6 is thus dependent upon value of ca acity C.

Applicant has pro need an amplifier accordlng to this invention which will transmit pictures at commercial speeds. B the a p" plicant has produced an amplifier having an inherent'capacity C (see Fi 6 of approximately 280 mmf. which is ca a le of amplifying without appreciable istortion, current changes at the rate of 1,000 per second usinga resistance of 200,000 ohms. The effect of condensers C C and C is substantially negligible, the combined effect being that of a condenser having a capacity value of only 20 mmf.

An amplifier with a capacity C of 500 mmf. would be suitable for transmission of pictures at a scanning speed of 10 inches per second which involves changes of current at the rate of 500 per second.

In order to produce such an amplifier, applicant has departed from standard amplifier design and has arranged the apparatus elements in the manner shown in Figs. 2 t0 5.

All of the equipment is mounted in a box 40 which is mounted on supports 41, 41 y means of soft rubber cushions 42, 42. Photoelectric cell 5 is mounted inside of a metal tube 43 which forms a part of the amplifier shield. The inside of the box is provided with a shield S. The apparatus elements such as electron discharge devices, potentiometers, rheostats and meters are mounted on a front panel as indicated. The batteries are mounted in battery boxes inside the shield as indicated in Figs. 3 to' 5, inclusive. Battery sections 11, 12 and 17 are located in battery box 44. The batteries in the output circuits of devices 6 and -7 are mounted in the other two boxes,

-45 and 46. Battery 19 is mounted on a the resistance, Y

, panel (has close as grid of as.

the-batteries in box 44 and t e shield S to a value of about 20 mmf. they are located with respect to the shield according to the dimensions a to 7', respectively. The location of the other batteries }is also shown. The values of these dimensions are as follows:

By the arrangement of apparatus elements hereinbefore described and departing from the well established rules of amplifier design, appliczint has produced an amplifier for the, small currents of a photoelectric cell produced during the process of picture transmission at speeds suitable for commercial working. He has accomplished this result by so arranging the elements that the inherent capacity of the system effectively in shunt of the input coupling resistance of the first amplifier stage is of very small value so as not to have a deleterious efiect upon the wave shape of the currents flowing in the photoelectric cell.

Modifications of the exact arran ements as hereinbefore described are possib c without departing from the invention as defined in the appended claims.

What is claimed is: Y

1. In an amplifier, a three-electrode electron discharge device, a high resistance bridged across the input circult of said device, means to cause small currents to flow through said resistance, and means com risin g a shield to maintain the capacity in s unt of said resistance at a value not substantially greater than 500. mmf.

2. In an amplifier, an electron discharge device having a grid, a cathode and an anode, a resistance of the order of 200,000 ohms connected between said gridand cathode, means to cause small currents having frequency components from zero up to several hundred cycles per second to flow through. said resistance, and means comprising a shield to maintain the capacity in shunt of said resistance at a value not substantially greater than 280 mmf.

3. In a photoelectric transmission system, a photoelectric cell, an amplifier havin an input and an output circuit, a modu ator having an input and an output circuit, a coupling between the input clrcuit of said amplifier and said cell for transmitting without distortion currents having fre quency components from .zero to several hundred cycles per second, a source of. po-

tential connected in the output circuit of said amplifier, and means to connect the input circuit of said modulator across a portion only of said source of potential.

4. In a photoelectric transmission system, a photoelectric cell, a three-electrode electron discharge amplifier having an input and an output circuit, a three-electrode electrondischarge modulator having an input and an output circuit, a source of potential and a resistance connected in series with said photoelectric cell across which resistance is connected the input circuit of said amplifier, 1

. of which is connected the input circuit of said modulators 5. In an amplifier, an electron discharge device having a grid, a cathode and an anode, a high resistance connected between said grid and cathode, a photoelectric cell-having a light sensitive cathode andan anode, a battery connected between saidgrid and the anode of said photoelectric cell, a low resistance connection between the cathode of said photoelectric cell, and the cathode of said electron discharge device, and means to maintainthe inherent capacity in shunt of said resistance at a value not substantially greater than 500 mmf.

6. In an amplifier, an electron discharge device having a grid, a cathode and an anode, a high resistance connected between said grid and cathode, a photoelectric cell having a light sensitive cathode and an anode, a battery connected between said grid and the anode of said photoelectric cell, a low resistance connection between. the cathode of said photoelectric cell and the cathode of said electron discharge device, and means to maintain the inherent capacity in shunt of said resistance at a value not substantially greater than 280 mi.

7. In an amplifier, an electron discharge device having a grid, a cathode and an anode, a high resistance connected between said grid and. cathode, a photoelectric cell having a light sensitive cathode and an anode, a batter connected between said rid and the ano e of said photoelectric cel, a

low resistance connection between the oath ode of said photoelectric cell and the cathode of said electron discharge device, and means to maintain the inherent capacity in shunt of said resistance at a value not substantially low resistance connection between the oathode of said hotoelectric cell and the oath ode of said e ectron discharge device, means to maintain the inherentcapacity in shunt of said resistance at a, value'not substantially greater than 280 mini, and means to vary the light in said photoelectric cell to produce currents having com onent frequencies extending over a range rom zero to 1,000

cycles per second.

9. In an amplifier, an electron discharge device having a grid, a cathode and an anode:i a high reslstance connected between ode of said sai grid and cathode, a photoelectric cell having a light sensitive cathode and an anode, a batter connected between said grid and the ano e of said photoelectric cell, a low resistance connection between the cathhotoelectric cell and the oathode of said e ectron discharge device, a metallic shield substantially surrounding said equipment,and means to space the equi went from said shield in order to maintain the capacity effectively in shunt of said resistance at a value not substantially greater than 500 mmf.

10. In an amplifier, an electron discharge device having a grid, a cathode and an anode, a high reslstance connected between said grid and cathode, a photoelectric cell 7 having a light sensitive cathode and an anode, a batter connected between said grid and the ano e of said photoelectric cell, a

low resistance connection between the oathode of said photoelectric cell and the cathode of i said electron discharge device, a shield substantially surrounding said equipment and means to locate said battery at least 1 inches from said shield.

In witness whereof, I hereunto subscribe my name this 30th day of April A. D. 1924.

DONALD M. TERRY. 

